Field of the Invention
The present invention relates to a method and a device for digitally measuring an analog voltage that varies over a predetermined voltage range, and more particularly to the use of such a device for measuring a temperature.
In automotive electronics, it is often necessary to measure the temperature of certain fluids, oil or coolant, or devices such as a catalytic converter for oxide reduction of the exhaust gases from a motor vehicle, or an oxygen sensor used in a device for regulating the air and fuel mixture supplied to the engine of such a vehicle. Currently, a suitable temperature pickup furnishes an analog electrical signal to an analog/digital converter, which furnishes a digital expression of the temperature measurement that can be used by a computer that forms part of an open- or closed-loop control unit provided in the vehicle.
The signal output by the pickup is normally an electrical voltage. When the temperature monitored becomes the subject of closed-loop control, this control keeps the signal within a predetermined voltage range. Thus oxygen sensors are known that are associated with a heating resistor actuated in such a way as to maintain the sensor temperature within a very narrow temperature range, for example 650.degree. to 750.degree. C., in which range the signal output by the sensor can be used. If the sensor temperature is evaluated on the basis of the temperature of the heating resistor, whose resistance is a function of that temperature, then the voltage picked up at its terminals to do so also varies within a range whose values are fixed by the limits of the aforementioned temperature range.
It has also been desirable for the temperature of the catalytic converter, around an optimal operating temperature of 850.degree. C., for example, to be known by a computer with a fixed precision of several percent. If a thermocouple is used to do so, then its output voltage varies in a corresponding manner within a predetermined voltage range.
The problem then arises of the precision of the digital measurement of the image voltage of the temperature observed. This precision is a function of the reference voltage V.sub.ref of the converter and of the number of bits n of the measurement N that it furnishes. In the aforementioned applications, measuring the voltage is of interest only within a predetermined voltage range, which a priori does not correspond to the dynamics of the digital measurement chain used, which depends on parameters (V.sub.ref, n) that are independent of this range.